Nucleoside triphosphate analogs and oligonucleotide analogs containing nuclease-resistant (3'-5') linked methyl phosphonate internucleoside linkages (NOP(O)(CH3)ON) will be studied for their ability to serve as substrates for enzymes involved in DNA synthesis and repair. The alpha-methyl phosphonate analog of thymidine triphosphate (pppT) will be tested as a substrate for DNA polymerase using polydeoxyriboadenylic acid and phi X174 DNA as templates. Oligonucleoside methyl phosphonates with base sequences complementary to selected polynucleotides and to regions of phi X174 DNA and Rous sarcoma virus RNA will be tested for their ability to initiate in vitro DNA synthesis at specific regions on their complementary templates catalyzed by DNA polymerase and reverse transcriptase. Methyl phosphonate modified polynucleotides produced in their studies will be tested as templates for in vitro DNA synthesis. In addition, oligonucleotide duplexes having regions which contain either phosphodiester, methyl phosphonate or ethyl phosphotriester linkages will be prepared. These duplexes will be tested as substrates for the joining enzyme, DNA ligase. The resulting sealed duplexes will contain the Hind III restriction enzyme site, A yields AGCTT as a precipitate and will be tested as substrates for Hind III. These experiments will make important contributions to three areas of nucleic acid biochemistry: (1) Experiments involving methyl phosphonate modified oligo- and poly-nucleotides will assess the effects that backbone modification could have on cellular DNA synthesis both at the substrate/initiator and template levels. (2) The effect of backbone modification on the interaction of ligase and restriction enzymes with modified nucleic acid duplexes will help define some of the parameters such as size of recognition site involved in their protein nucleic acid interactions. (3) Since oligonucleoside methyl phosphonates may be taken up by cells in culture, they may find use as probes of DNA synthesis in living cells.